Artificial Rock Walls In Singapore Biology Essay

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Rock walls are one of the common sites found along Singapore coastline. This can be viewed as a main disruption body to the marine coastal environment. In this study, the diversity and distribution patterns of these intertidal communities will be examined on ten similar material made rock walls at several coastal parks around Singapore coastlines as well as off shore islands. It will also compare biodiversity and community assemblages after the first monitoring exercise by Lee, 2009. In addition, this study will assess if the angle of slope of the rock walls, a physical factor, may affect community assemblages from different location. Analyses revealed that there are significant differences between various angle slopes in the area of abundance, species diversity, evenness and richness. Comparison studies with Lee, 2009 is not conclusive as both sampling methods varied with different tidal zones, however, similarity can be seen in some assemblages from the past 10 years in the same shore height site of the rock walls. On the whole, the results gathered in the studies cannot fully support the hypothesis that the physical factor of the angle of the slope can affect the community assemblages on artificial intertidal rock walls. There are many factors and mechanisms that can affect the growth and structure of the assemblages in the artificial rock walls. These should be examined in an evaluating perspective, as not relying on existing data found on the rock walls to conclude that they are alternative marine habitat.

INTRODUCTION

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Currently, one of the world conservation issues is our marine biodiversity which has been negatively impact in the coastal areas. The enormous depletion can be due to the large number of interface uses of the costal habitat such as heavy industry, recreational, seaports and even residential (Gray 1997). Around the world, there are many coastal area and shorelines that have been converted for the increasing demand of residential, commercial, transportation and tourism. There is also a relationship of the increase in shoreline alteration and the increase of population along the shoreline as well (Bulleri & Chapman, 2010). There are two effects in the marine environment of protecting the shoreline, which is primarily disrupting the pre-existing habitat and the introduction of foreign, new habitat. These effects are well documented previously (Bulleri & Chapman, 2010). Commonly in Singapore, seawalls are constructed among sandy sediments for the prevention of erosion, thereby providing stabilization and protection from currents and waves. However, it can change both the physical habitat and biological community. It was observed that different type of shoreline protection structure such as seawalls, breakwaters, groins and riprap have different effect on the biodiversity and evenness of marine species (Davis, Levin et al. 2002; Bacchiocchi and Airoldi 2003; Bulleri and Chapman 2004). Seawalls are one of the simplest shoreline protection structures, which in this case made up of smooth vertical concrete slabs. In the intertidal zone of the seawalls, it was observed to have fewer taxa when compared to the natural rocky shores. This is because of the lack of crevices which are known to be an important shelter for mobile and sessile invertebrates (Petraitis and Dudgeon 2005; Moreira, Chapman et al. 2007). Physical characteristics such as low surface area that lead to increase of stress and competition, lack of shading and sloping surfaces also may influence the habitat recruitment of sessile invertebrates (Chapman and Blockley 2009). In Moreria studies, it was observed there are some similar taxa from natural rocky shores are also found in the intertidal zone of the seawalls (Moreira, Chapman et al. 2006).

The objective of this study is to evaluate the abundances, distribution and diversity of various seawall designs around Singapore that incorporate angle of slope. Based on Lee studies on Singapore seawalls, we shall examine 4 locations ranging from coastal to offshore islands (Lee et al. 2009). From this study, we can find out the potential seawall design with the optimum angle to improve the ecology of intertidal zone in an artificial structure environment.

RESULTS

A total of 33 marine species were observed at the allocated inter-tidal zone were observed across 17 rock walls around Singapore (Table 1). Comparing the findings with Lee, Tan during 2002, some of the species are lost from current observations. 4 new marine species were observed in Pasir Ris, East Coast Park and Labrador Park respectively. A total of 51 invertebrate taxa were documented across the 4 locations. Intertidal fauna such as Balanus sp., Serpulidae and Littoraria sp. were observed from all survey locations, while some species such as Grapsidae, T.gradata, P.duclosiana, Perna virdis, Trapeezium sp., Irus sp., Cerithium coalium, Murex aduncospinosus and Polifera were only observed in a single location (Table 1).

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There were obvious differences and variation in the intertidal communities which are examined across the rock walls in 4 locations which display similarity phenomenon with Lee, Tan observations in 2002. Total abundances with respect to the mean angle of slope was plotted to show pattern growth among the rock walls at different locations (Figure 1). With the highest abundance individual count at Pasir Ris of above ten thousands at the angle of 27.54 to as little as below two thousands at East Coast Park at the angle of 33.56. The highest abundance counts fall on the range of angle 27 to 30 degree which is significant difference across other slope angles (Table 2).

Table 1. A list of intertidal organism found across 10 seawalls around Singapore comparing observations with Lee, Tan et al. 2009. Shaded cross indicate species matched. Abbreviation used: PR, Pasir Ris Park; ECP, East Coast Park; MP, Marine Parade; SJ, Saint John Islands; LP, Labrador Park.

Species

Location

PR

PR (Lee, 2009)

ECP

MP (Lee, 2009)

SJ

SJ (Lee, 2009)

LP

LP (Lee, 2009)

Balanus sp.

X

X

X

X

X

X

X

X

Chthamalidae

X

X

X

X

Tetraclita spp.

X

X

X

Grapsidae

X

Ligia sp.

X

X

X

X

Serpulidae

X

X

X

X

X

X

X

X

Patelloida sp. A

X

X

X

X

X

Patelloida saccharina

X

X

X

X

X

X

X

Cellana radiate (Born, 1779)

X

X

X

Turbo bruneus (L., 1758)

X

X

X

Trochus maculatus (L., 1758)

X

X

X

X

Monodonta labio (L., 1758)

X

X

Euchelus sp.

X

X

Nerita albicilla (L., 1758)

X

X

N. chamaeleon (L., 1758)

X

X

N. undata (L., 1758)

X

X

Echinolittorina malaccana (P., 1847)

X

X

X

X

X

X

X

E. vidua (Gould., 1859)

X

X

X

X

X

X

E. melanacme (Smith, 1876)

X

X

X

X

Littoraria spp.

X

X

X

X

X

X

X

X

Peasiella spp.

X

X

X

X

X

Vermetus cf. alii

X

X

X

Gyrineum natator (Roding, 1758)

X

Lataxiena bimucronata (Reeve, 1846)

X

X

X

X

Morcula musiva (Kiener, 1835)

X

X

M. fusca (Kuster, 1858)

X

X

X

M. margariticola (Broderip 1833)

X

X

X

X

T. clavigera (Kuster, 1858)

X

X

X

X

X

T. jubilaea (Tan & Sigurdsson, 1990)

X

T. rufotincta (Tan & Sigurdsson, 1996)

X

X

X

T. gradata (Jonas, 1846)

X

Pictocollumbella ocellata

X

X

P.duclosiana

X

Siphonaria arta (Q & Gaimard, 1833)

X

X

X

X

X

X

S. guamensis (Q & Gaimard, 1833)

X

X

X

X

X

X

X

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S. javanica (Lamarck, 1819)

X

X

X

X

X

Septifer excisus (Wiegmann, 1837)

X

X

Perna virdis (L, 1758)

X

Xenostrobus sp.

X

X

X

Saccostrea cuccullata (Born, 1778)

X

X

X

X

Ostrea sp.

X

X

X

X

X

Anomia sp.

X

X

X

X

X

Trapeezium sp.

X

Irus sp.

X

Chiton

X

X

X

X

X

Hydroida

X

X

X

X

Anemone

X

X

Cerithium coalium

X

Murex aduncospinosus

X

Polifera

X

Thais bitubercularis

X

X

Species diversity as being determined by using Shannon-Weiner diversity index was fairly diverse across the different mean slopes groups which exhibit the index from 0.5 to 0.7 with a significant figure of 0.001 (Table 2). The assemblages on the rock walls also fairly exhibited fairly differences upon its community evenness with value range from 0.3 to 0.5 which also implies the same to species richness with value from 0.2 to 0.5 with significant figure of 0.028 and 0.007 respectively (Table 2). The rock walls assemblages at the angle mean group of 55-60 degree were characterized by low total abundance and relative high species diversity (Figure 1, 2), indicating exceptionally marked patterns of low dominance by a big range of species. Low evenness has also seen in mean angle group 20-25 and 35-40 which also indicate the abundance level is not equally distributed across the species (Figure 2).

SJ

LP

SJ

LP

LP

PP

LP

PP

ECP

LP

PP

SJ

ECP

ECP

ECP

SJ

SJ

Figure 1.Total abundance (as total number of individual at each slope angle of each rock wall). See Table 1for location abbreviation.

At each mean angle group, the rock walls communities were numerically dominated by at least one to two main taxa, which account at least more than 5 percent of the total abundance at those groups. Balanus spp. seems to be the most dominant among the rest which are observed in all angle groups except 35-40 degree group (Figure 3). Overall, Siphonariid limpets and barnacles were among the top three rank abundances display in all mean groups. All other taxa not display in figure 3, occurred at considerable low abundance, usually less than twenty individuals of each taxa at each surveyed rock walls across all locations. When comparing the distribution of abundances, diversity, richness and evenness with different slope angle groups, results show that there is significant differences where P <0.05 among the 168 samples across the 4 locations (Table 2).

Figure 2. Measure of species diversity, evenness and richness at different group of angle slope for 4 study locations.

Nerita undata

Balanus spp.

Siphonaria atra

Balanus spp.

Siphonaria quamensis

Xenostrobus sp.

Balanus spp.

Saccostrea cuccullata

Siphonaria quamensis

Siphonaria javanica

Patelloida saccharina

Balanus spp.

Tetraclita spp.

Saccostrea cuccullata

Siphonaria quamensis

Figure 3. Rank abundance graph for the 4 study locations in terms of mean slope angle grouping. Data shown is only for taxa that has are the first 3 ranking of the total abundance in regards to the groups.

Table 2. Statistic non-parametric test results for the analyses of differences across different slopes mean angle groups. *P <0.05

Sources

Test

Total N

d.f.

Test Statistics

Sig.

Distribution of abundances across mean slope angle

NP (Kruskal-Wallis)

168

4

23.029

< 0.05*

Distribution of species diversity across mean slope angle groups

NP (Kruskal-Wallis)

168

4

19.627

0.001*

Distribution of species richness across mean slope angle groups

NP (Kruskal-Wallis)

168

4

10.681

0.028*

Distribution of species evenness across mean slope angle groups

NP (Kruskal-Wallis)

168

4

13.950

0.007*

The SPSS analysis results showed that the assemblages of various group angles lies across four locations were significant different, the nMDS ordination plots distinguished between the group angles were also reflected in the same manner. However the assemblages varied significantly among the locations within the same location (Figure 4). Assemblages at Pasir Ris Park and St John Island were generally more similar than assemblages than the rest of the locations with less dispersion among locations and sites in the MDS ordination plots (Figure 2). The MDS plots also showed that there is some similarity at the mean slope group of 55-60 angle where the rest of the mean slopes are quite dispersed.

Figure 4. Two-dimensional MDS plot of centroids of the replicate samples of species no, abundances, diversity, richness and evenness in: a, rock walls location and b, mean slope angle.

DISCUSSION

This study show that the rock walls in Singapore regardless of different slope angle is able to support a relatively high diversity of intertidal assemblages which reaches a total of 51 species at the intertidal zone of the upper mid shore (-0.5 to 0.5 m). The type of species observed from these artificial rock walls in this study show similarity with the previously recorded studies of the artificial rock walls in Singapore conducted in 2002 and 2003 (Lee, et al. 2009). Their studies show that at most of the 12 study locations, the species diversity range from 2.25 to 3.25. Highest abundance can reach up to 5500 individuals of S.guamensis found in Pasir Ris Park. However in this study, species diversity ranges from 0.45 to 0.8 across 4 study locations are observed (Figure 2). Highest abundance alone reached up to 8000 individual of Balanus sp. found in Pasir Ris Park. This cannot be represented that there is a significant decrease in the species diversity and increase in abundances of the dominant species found in a single location. This can be partially due to the different sampling area of the rock walls from these 2 studies. In Lee studies, they sampled 4 different tidal sections of the rock walls which include the Supralittoral of > 1.1 m above the mean sea level. Compared to this study, the sampling area only consists of one tidal section of Upper Midshore -0.5 to 0.5m. Therefore if based on these two different sampling approaches, the numbers of intertidal fauna will be much different and Lee studies will be assumed to be higher species diversity. Due to this reason, the observation of the dominant species will also be different. For example in Pasir Ris Park location, Lee studies revealed that there are 5500 individuals of S.guamensis and these observations account the total of 4 tidal sections sampled. In this study, the sampling time during this location is morning and S.guamensis, a mobile invertebrate would have moved beyond the upper midshore zone for food hunting. Balanus sp. on the other hand, is an immobile invertebrate, at its highest abundance were shown in the tidal zone.

Even though we cannot conclude that if there is a significant change in both of these studies that were 10 years apart, this study reveal that there are still some species found in the artificial rock walls which comes with a species match with Lee studies (Table 1). 63% of the same species located in the upper midshore zone were found to be the same when compared with Lee studies (although there is no data from Lee studies to show that the species found in which different tidal zone). It seems that artificial rock walls are able to support a subset group of those assemblages which can be found in natural habitats (Bulleri & Chapman, 2004). From this study alone, mobile organisms were found relatively low (Chapman, 2003). This can be explained where the rock wall surveyed were uniformity constructed which there is a lack of cracks, holes, crevices and rock pools. These areas consist of some specialized organism living in this habitat (Moreira et al., 2006).

This study also demonstrates that the intertidal assemblages on the artificial rock walls are strongly influence by the slope angle of the walls. From the abundance data, most of the intertidal fauna can be found within 27-28 degree slope with the exceptional of 27.58 degree in LP site. The inconsistent of the abundance data can be explained due to the fact of the different sampling site. Labrador Park was recently construction of the boardwalk along the shore in 2011 (MPA notice, 2011) and this may result the destruction of the existing fauna in the artificial rock walls. It is also evidence in this study that the highest abundance lies at Pasir Ris Park and East Coast Park. Species diversity index of mean group 25-30 scored up to 0.7 while mean group of 55-60 reaches 0.8. This also can be explained that St John Island is the only place in this study to be survey with a mean slope angle between 55-60 degree. This site has a particular higher number of species when compared to mainland sea walls surveys. As mentioned in Lee studies, there is a different in the assemblages from the mainland and offshore islands in the area of abundances in a small subset of species (Lee and Sin, 2009). The abundance of the species may be lower than the mainland which can also explained due to the poorly nutrient water circulating from the open sea.

The dominant species in this study, Balanus spp. has probably no limitation and is able to grow in a wide range of slope angle. However, the highest abundance is found in 25-30 degree. The settlement and attachment can determine the survival of barnacles in the marine system and the slope angle can play an important factor (Mittas et al., 2001). More studies are required to understand the mechanism of barnacle settlement on different angles. Siphonaria species are also one of the commonly found organisms among all locations surveyed. However, some species such as the bivalves Xenostrobus sp and Perna viridis were commonly observed in Pasir Ris Park. This result can also be seen in Lee experiment in 2004. It can be concluded that there are different water bodies surrounding Singapore and its islands with distinct different environmental conditions (Lim, 1983).

The distribution of the intertidal organisms found across the different mean slope angle has been affected by a combination of physical and biological factors. As explained in Chapman, different stress such as temperature, desiccation and exposure to air tend to increase as the height of the shore increase (Chapman and Underwood, 1996). As the slope of the rock walls get steeper, the height of the shore will also tend to increase which also implies similar effect mentioned. Fishes as predators may also affect their distribution during high tide, where gentle slope may increase the chance of being a prey (Levings and Garrity, 1983). Species distributed in steeper angle slope between 55-60 degree, are most likely being influenced by biological factors and behavioral adaptations towards the physical condition (Garrity, 1984). There are actually a lot of unknown physical factors which somewhat can affect the rock walls. Exposure to wave and frequent boat wakes may be able to reduce the surface temperature of steeper slope rock walls when compared to a gentler slope. However it can also increase physical impact on the organisms and result in higher mortality (Whorff et al., 1995). This is evident in Saint John Island, where total abundance is particularly low in steeper slope rock walls where they are situated near to the jetty and commercial vessels.

Artificial rock walls structures indeed can support an alternative habitat for marine organisms based on this study. The process on how these assemblages settled on this artificial rock walls remains unclear to many scientists and this is one of the important factor to manage sustainable coastlines. The data collected in this study indeed represent a degree of significant differences with different slope angle of the rock walls. It is also evident that there is an optimal angle of the slope that is able to encourage most of the assemblages to settle and grow. However, the data collected do not represent a clear trend between the relationship of the slope angle and the assemblages. Collectively, the findings of this study do not fully support the hypothesis model to improve the ecology of intertidal zone in an artificial structure environment with respect to the angle of the slope. This can be due to the variability across different locations surveyed that are influenced by physical and chemical environment that are explained by previous rock walls studies (Lee et al., 2009). More studies are required to evaluate the relationship between the environment and the differences within those assemblages found on the artificial rock walls.

ACKNOWLEDGEMENTS

I would like to express my sincere appreciation to my students from ITE: Chia Loon, Eugene Chieng, Wei Xuan, Janick Choo, Ying Jie, Ciyue and Kevan Chua who accompany me on early morning and noon doing field work with me. Your contribution and pleasant company made this project both achievable and fun. The support from NSSE Department Mr. Chien Houng and the NIE undergraduates for this project is greatly appreciated. I am also grateful to my project supervisor, Dr Beverly Goh who guided and advised me to make this project possible.